A　single-screw　expander　(SSE)　is　a　type　of　volumetric　expander　that　can　be　applied　to　power　generation,　which　is　beneficial　for　energy　conservation　and　environment　protection.　Investigating　the　SSE　leakage　characteristics　is　necessary　because　leakage　has　a　significant　influence　on　the　SSE　performance.　This　study　provides　theoretical　and　experimental　analyses　of　the　SSE　internal　leakage　with　compressed　air　as　the　working　fluid.　Both　mathematical　model　of　different　leakage　paths　and　leakage　model　have　been　established.　The　simulation　model　was　verified　by　experimental　data　at　internal　volume　ratios　of　2.95　and　shows　a　maximum　deviation　of　3.63%.　Results　show　that　fitting　clearance　height　and　rotational　speed　could　clearly　influence　the　volumetric　efficiency　and　power　output.　The　diameters　of　both　the　screw　and　the　gate　rotor　could　also　significantly　influence　the　volumetric　efficiency.　The　volumetric　efficiency　decreased　with　the　increase　in　internal　volume　ratio.　(c)　2017　Elsevier　Ltd　and　IIR.　All　rights　reserved.